Organic electro luminescent display panel and fabricating method thereof

ABSTRACT

There is provided an organic EL display panel having a pixel circuit that increases the panel&#39;s aperture ratio by an improved structure of a unit pixel. The present invention discloses an organic EL display panel including at least one pixel circuit for driving each pixel, wherein the pixel circuit comprises at least one organic light emitting element, at least two thin film transistors and at least one capacitor per pixel. The thin film transistors respectively comprise a gate electrode and a semiconductor layer in which a channel region, a source region and a drain region are formed. The conductive layer contacted on the gate electrode of one of the thin film transistors is coupled to another thin film transistor and at least one capacitor.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2004-0037278, filed on May 25, 2004, which is herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electro luminescent (EL)display panel, and more particularly to an organic EL display panel inwhich an aperture ratio is increased by improving a structure of a unitpixel.

2. Description of the Background

An organic EL display uses light emitted from an electrically excitedorganic light emitting diode OLED to display characters or images.Electrons, supplied from a cathode, and holes, supplied from an anode,recombine to excite the organic material. Generally, the organic lightemitting diode OLED includes an anode electrode layer, an emitting layer(EML) for facilitating supply of electrons and holes, an electrontransport layer (ETL), a hole transport layer (HTL) and a cathodeelectrode layer.

Methods for driving organic EL displays are generally classified aspassive or active matrix methods. The active matrix method may include avoltage programming method and a current programming method, dependingon the form of a signal that charges a voltage into a capacitor andmaintains the charged voltage.

FIG. 1 shows an equivalent circuit of a conventional active matrix-typeorganic EL display driven by the voltage programming method. Referringto FIG. 1, pixels may be arranged in the form of a matrix defined byintersections among scan lines S₁ to S_(n), data lines D₁ to D_(m), andpower lines V₁ to V_(m), and each pixel may include a switching thinfilm transistor ST, a driving thin film transistor DT, and a storagecapacitor C_(ST).

In each pixel, a source electrode, a gate electrode, and a drainelectrode of the switching thin film transistor ST are coupled to datalines D₁ to D_(m), scan lines S₁ to S_(n), and a gate electrode of thedriving thin film transistor DT, respectively. The storage capacitorC_(ST) is coupled between the drain electrode of the switching thin filmtransistor ST and a power lines Vn. A source electrode and a drainelectrode of the driving thin film transistor DT are coupled to thepower line Vn and an organic light emitting element OLED, respectively.The drain electrode of the driving thin film transistor DT may beelectrically connected an anode electrode of the organic light emittingelement OLED. A cathode electrode of the organic light emitting elementOLED may be supplied with a common voltage for all pixels.

When the switching thin film transistor ST turns on from a selectionsignal applied to its gate electrode, a data voltage from the data linesD₁ to D_(m) is applied to the gate electrode of the driving thin filmtransistor DT. Then, in response to a voltage V_(GS) charged in thestorage capacitor C_(ST) between the gate electrode and the sourceelectrode of the driving thin film transistor DT, a current I_(OLED) mayflow through the organic light emitting element OLED via the drivingthin film transistor DT, thereby emitting light from the organic lightemitting element OLED.

In the voltage programming method as described above, a problem mayarise in that brightness of an organic EL display panel may not beuniform due to deviation in characteristics of driving thin filmtransistors, such as threshold voltage or channel mobility.

Accordingly, complementary circuits for correcting this deviation incharacteristics have been proposed. However, increasing the number ofthin film transistors may decrease the pixel's aperture ratio.

On the other hand, assuming that a current source for supplying acurrent to pixel circuits is uniform for the entire panel, i.e., alldata lines, the current programming type organic EL display may obtainuniform display characteristics even when the pixels' driving thin filmtransistors have a non-uniform voltage-current characteristic.

FIG. 2 is a pixel circuit showing a conventional current programmingmethod for driving an organic EL display, where a single pixel is shown.Referring to FIG. 2, a driving thin film transistor DT is coupled to anorganic light emitting element OLED to supply a current for emittinglight, and a data current I_(DATA), which is applied through a switchingthin film transistor ST1, controls the amount of current flowing throughthe driving thin film transistor DT.

When switching thin film transistors ST1 and ST2 are turned on by aselection signal from a scan line S_(n), the driving thin filmtransistor DT is diode connected. Consequently, a storage capacitorC_(ST) is charged to a voltage as a current flows through it. Namely, apotential of a gate electrode of the driving thin film transistor DTdrops, thereby causing the current to flow from a source electrode ofthe driving thin film transistor DT to a drain electrode of the drivingthin film transistor DT, so that the storage capacitor C_(ST) is chargedto the voltage corresponding to the data current I_(DATA) for settingbrightness. Next, the switching thin film transistors ST1 and ST2 areturned off, and a thin film transistor ET, which is coupled to anemission control line E_(n), is turned on. Then, power is supplied froma power supply line VDD, and a current I_(OLED) corresponding to thecharged voltage of the storage capacitor C_(ST) flows through theorganic light emitting element OLED to emit light with a presetbrightness.

However, since the current I_(OLED) flowing through the organic lightemitting element OLED may be minute, and the voltage range of the dataline D_(m) may be wide, it may take a relatively long time to charge aparasite capacitor of the data line.

Additionally, increasing the number of thin film transistors disposed ina unit pixel may significantly reduce the aperture ratio, which maydeteriorate brightness. Furthermore, the display's lifetime may bereduced if the pixel circuits are driven with a high current.

SUMMARY OF THE INVENTION

The present invention provides an organic EL display panel wherein anaperture ratio is increased by improving an arrangement structure of aunit pixel.

Additional features of the invention will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention.

The present invention discloses an organic EL display panel including atleast one pixel circuit for driving each pixel, wherein the pixelcircuit comprises at least one organic light emitting element, at leasttwo thin film transistors and at least one capacitor per pixel. The thinfilm transistors respectively comprise a gate electrode and asemiconductor layer in which a channel region, a source region and adrain region are formed. The conductive layer contacted on the gateelectrode of one of the thin film transistors is coupled to another thinfilm transistor and at least one capacitor.

The present invention also discloses a method for fabricating an organicEL display panel, comprising the steps of: forming a commonsemiconductor layer for at least two thin film transistors on aninsulation substrate, forming a gate insulation film covering thesemiconductor layer, forming gate electrodes for the at least two thinfilm transistors on the gate insulation film, forming a interlayerinsulation film covering the gate electrode, forming a contact hole forexposing the gate electrode of one of thin film transistors, and forminga conductive layer inside the contact hole and the interlayerinsulation.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the is principles of theinvention.

FIG. 1 is an equivalent circuit showing a conventional activematrix-type organic EL display.

FIG. 2 is a pixel circuit diagram showing a conventional currentprogramming method for driving a conventional organic EL display.

FIG. 3 is a schematic diagram showing a structure of an organic ELdisplay according to exemplary embodiments of the present invention.

FIG. 4 is a schematic diagram showing an organic EL display panelaccording to an exemplary embodiment of the present invention.

FIG. 5 is an equivalent circuit showing a pixel of FIG. 4.

FIG. 6 is a diagram showing an arrangement of the organic EL displaypanel according to an exemplary embodiment of the present invention.

FIG. 7 is a sectional view taken along line I–I′ of FIG. 6.

FIG. 8 is a sectional view taken along line II–II′ of FIG. 6.

FIG. 9 is a sectional view taken along line III–III′ of FIG. 6.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The following detailed description shows and describes exemplaryembodiments of the present invention. As those skilled in the art wouldrecognize, the described exemplary embodiments may be modified invarious ways, all without departing from the spirit or scope of thepresent invention.

In the drawings, illustrations of elements having no relation with thepresent invention are omitted in order to prevent the subject matter ofthe present invention from being unclear. In the specification, the sameor similar elements are denoted by the same reference numerals eventhough they are depicted in different drawings. As used herein, acoupling between one element and another element includes not only adirect coupling therebetween but also an indirect coupling therebetweenwith other elements interposed therebetween. In addition, forming oneelement such as a layer, a film, a region or a plate on another elementincludes not only forming the former immediately above the latter butalso forming the former above the latter with other elements interposedtherebetween.

FIG. 3 is a schematic diagram showing a structure of an organic ELdisplay according to an exemplary embodiment of the present invention.

Referring to FIG. 3, the organic EL display may include a display panel100, a scan driver 200, and a data driver 400. The display panel mayinclude a plurality of scan lines S₁ to S_(n), a plurality of emissioncontrol lines E₁ to E_(n) and a plurality of boost control lines B₁ toB_(n) extending in a row direction, and a plurality of data lines D₁ toD_(n) and a plurality of power lines VDD extending in a columndirection. A plurality of pixels 105 may be formed between intersectionsof these lines.

The pixels 105 may be formed in a pixel region defined by two adjacentscan lines S_(k-1) and S_(k) and two adjacent data lines D_(k-1) andD_(k), and they may be driven by signals transmitted from the scanlines, the emission control lines E₁ to E_(n), the boost control linesB₁ to B_(n), and the data lines.

Additionally, the scan driver 200 may sequentially transmit a selectionsignal to select a corresponding scan line, which allows application ofa data signal to that scan line's pixels, and it may sequentiallytransmit an emission control signal to the emission control lines E₁ toE_(n) to control emission of the organic light emitting element OLED.

The scan driver 200 may also apply a boost signal to pixels of acorresponding boost control line B1 to Bn to increase the gate voltageof a driving thin film transistor using two capacitors (C1 and C2 inFIG. 4). Accordingly, a current supplied to the organic light emittingelement OLED may be set to a desired value.

In the mean time, the data driver 400 applies a data signal via the datalines D₁ to D_(n) to pixels of a selected scan line.

In this way, the scan driver 200 and the data driver 400 may berespectively coupled to a substrate of the display panel 100,respectively. Alternatively, the scan driver and/or the data driver maybe directly formed on a glass substrate of the display panel 100, orthey may be replaced with any driving circuit that may be formed on thesame layer as the scan lines, the data lines and the transistors on thesubstrate of the display panel 100. Additionally, the scan and/or datadrivers may be formed in the form of a chip on a tape carrier package(TCP), a flexible printed circuit (FPC), or a tape automatic bonding(TAB), which are bonded and coupled to the substrate of the displaypanel.

Next, an operation of the organic EL display will be described in detailwith reference to FIG. 4 and FIG. 5.

FIG. 4 is a schematic diagram showing the display panel according to anexemplary embodiment of the present invention, and FIG. 5 is anequivalent circuit showing a single pixel of FIG. 4.

Referring to FIG. 4 and FIG. 5, a display panel pixel circuit mayinclude a driving transistor M3, an emission transistor M4, a switchingtransistor M1, a diode transistor M2, an organic light emitting elementOLED, and two capacitors C1 and C2.

In more detail, the switching transistor M1 may be coupled between thedata line D_(n) and the gate electrode of the driving transistor M3, andin response to the selection signal from the scan line S_(n), ittransmits the current I_(DATA) from the data line D_(n) to the drivingtransistor M3. The diode transistor M2 may be coupled between the drainregion of the driving transistor M3 and the data line D_(n) todiode-connect the driving transistor M3 in response to the selectionsignal from the scan line S_(n).

Additionally, source and drain regions of the driving transistor M3 maybe coupled to the power line VDD and the drain region of the diodetransistor M2, respectively. A gate-source voltage of the drivingtransistor M3 may be determined according to the data current I_(DATA).

The second capacitor C2 may be coupled between the gate electrode andthe source region of the driving transistor M3 to maintain thegate-source voltage of the driving transistor M3 during a period oftime, and the first capacitor C1 may be coupled between the boostcontrol line B_(n) and the gate electrode of the driving transistor M3to adjust a gate electrode voltage of the driving transistor M3.

By coupling the capacitors C1 and C2 as shown in FIG. 4 and FIG. 5, avoltage of the first capacitor C1 may increase by an increase of a boostcontrol signal voltage (ΔV_(B)) applied from the boost control lineB_(n). Hence, the gate electrode voltage increase (ΔV_(G)) of thedriving transistor M3 may be obtained according to Equation 1.Therefore, by adjusting the increase of the boost control signal voltage(ΔV_(B)) corresponding to parasite capacitive components of thetransistors M1, M2 and M3, the gate electrode voltage increase (ΔV_(G))of the driving transistor M3 may be set to a desired value.

$\begin{matrix}{{\Delta\; V_{G}} = \frac{\Delta\; V_{B}C_{2}}{C_{1} + C_{2}}} & {{Equation}\mspace{20mu} 1}\end{matrix}$

Next, in response to the emission signal from the emission control lineE_(n), the emission transistor M4 supplies a current flowing through thedriving transistor M3 to the organic light emitting element OLED. Theorganic light emitting element OLED may be coupled between the emissiontransistor M4 and a reference voltage VSS to emit light corresponding tothe amount of current I_(OLED) flowing through the driving transistorM3.

Operation of the pixel circuit described above will be now described indetail.

First, the switching transistor M1 and the diode transistor M2 areturned on by the selection signal applied through the scan line S_(n).Accordingly, the driving transistor M3 is diode-connected, and the datacurrent I_(DATA) from the data line D_(n) flows into the drivingtransistor M3. Simultaneously, since the emission transistor M4 isturned off by the emission signal applied through the emission scan lineE_(n), the driving transistor M3 and the organic light emitting elementOLED are electrically insulated from each other.

At this time, Equation 2 shows the relationship between an absolutevalue of a voltage V_(GS) between the gate and the source of the drivingtransistor M3 and the data current I_(DATA) flowing through the drivingtransistor M3, and Equation 3 gives the gate-source voltage V_(GS).

$\begin{matrix}{I_{DATA} = {\frac{\beta}{2}\left( {V_{GS} - V_{TH}} \right)^{2}}} & {{Equation}\mspace{20mu} 2}\end{matrix}$

Where, β is a constant and V_(TH) is the absolute value of a thresholdvoltage of the driving transistor M3.

$\begin{matrix}{V_{GS} = {{V_{DD} - V_{G}} = {\sqrt{\frac{2I_{DATA}}{\beta}} + V_{TH}}}} & {{Equation}\mspace{20mu} 3}\end{matrix}$

Where, V_(G) is a voltage of the gate electrode of the drivingtransistor M3, and V_(DD) is a voltage supplied to the drivingtransistor M3 by a power line VDD.

Next, the switching transistor M1 and the diode transistor M2 are turnedoff by the selection signal of the scan line S_(n), and the emissiontransistor M4 is turned on by the emission signal of the emissioncontrol signal E_(n).

At this time, a voltage at a point of contact between the secondcapacitor C2 and the first capacitor C1 may rise by an increase of theboost control signal voltage (ΔV_(B)) applied from the boost controlline B_(n). Accordingly, the gate voltage V_(G) of the drivingtransistor M3 may rise by a coupling between the capacitors C1 and C2,and Equation 1 noted above gives the gate voltage increase ΔV_(G).

Since the gate voltage V_(G) of the driving transistor M3 increases byΔV_(G), the current I_(OLED) flowing through the driving transistor M3may be determined by the Equation 4. Namely, since the gate-sourcevoltage V_(GS) decreases by the gate voltage increase ΔV_(G), themagnitude of the current I_(OLED) of the driving transistor M3 may beless than that of the data current I_(DATA). Additionally, since theemission transistor M4 is turned on by the emission signal of theemission control line En, the current I_(OLED) of the driving transistorM3 may be supplied to the organic light emitting element OLED.

$\begin{matrix}{I_{OLED} = {{\frac{\beta}{2}\left( {V_{GS} - {\Delta\; V_{G}} - V_{TH}} \right)^{2}} = {\frac{\beta}{2}\left( {\sqrt{\frac{2I_{DATA}}{\beta}} - {\Delta\; V_{G}}} \right)^{2}}}} & {{Equation}\mspace{20mu} 4}\end{matrix}$

Equation 4 may be rearranged into Equation 5 to give the data currentI_(DATA). Equation 5 shows that the data current I_(DATA) may be set ata greater value than the current I_(OLED) flowing through the organiclight emitting element OLED.

$\begin{matrix}{I_{DATA} = {I_{OLED} + {\Delta\; V_{G}\sqrt{2\beta\; I_{OLED}}} + {\frac{\beta}{2}\left( {\Delta\; V_{G}} \right)^{2}}}} & {{Equation}\mspace{20mu} 5}\end{matrix}$

Hereinafter, a layout of the display panel according to an exemplaryembodiment of the present invention will be described with reference toFIG. 6, FIG. 7, FIG. 8 and FIG. 9. FIG. 6 shows an arrangement of thedisplay panel according to an exemplary embodiment of the presentinvention, and FIG. 7, FIG. 8 and FIG. 9 are sectional views taken alonglines I–I′, II–II′ and III–III′ of FIG. 6, respectively.

Referring to FIG. 6, FIG. 7, FIG. 8 and FIG. 9, pixels may be defined bya data line 110 extending in a first direction (the Y axis direction ofFIG. 6), a scan line 120 arranged in a second direction (the X axisdirection in FIG. 6) intersecting with the data line 110, a power line130 extending parallel to the data line 110 and spaced therefrom by aconstant interval, an emission control line 140 arranged in parallel tothe scan line 120, and a boost control line 150 arranged in parallel tothe emission control line 140 spaced therefrom by a constant interval.

Herein, the switching transistor M1, the driving transistor M3, thediode transistor M2 and the emission transistor M4 comprising the pixelcircuit may be disposed in a space between the scan line 120 and theemission control line 140. Accordingly, since the boost control line 150does not overlap with the elements comprising the pixel circuit, boostsignal distortion due to interference between the elements of the pixelcircuit may be prevented. Consequently, since the boost signal may bestably applied to the first capacitor C1, a current I_(OLED) may be moreprecisely supplied to the organic light emitting element OLED.

The thin film transistors M1, M2, M3 and M4 respectively comprise a gateelectrode 51, 52, 54 and 53 and a semiconductor layer 30 in which achannel region, a source region and a drain region are formed. Inpresent invention, a semiconductor layer 30 of the switching transistorM1, the diode transistor M2, the driving transistor M3 and the emissiontransistor M4 may be formed as a common layer.

More specifically, the switching transistor M1 may be formed at thevicinity of an intersection between the scan line 120 and the data line110. The source region of the switching transistor M1 is coupled to thedata line 110 through a contact hole h1, and the drain region of theswitching transistor M1 is coupled to the gate electrode 54 of thedriving transistor M3 through contact holes h2 and h3 via a conductivelayer 135. Herein, the drain region of the switching transistor M1 maybe coupled to the gate electrode 54 of the driving transistor M3 throughthe conductive layer 135 having a inverse “L” like shape in FIG. 6,thereby reducing an area in which the pixel circuit is disposed andincreasing an aperture ratio of the pixel, which will be described indetail later.

Additionally, the diode transistor M2 may be formed at the vicinity ofan intersection between the data line 110 and the emission control line140. The gate electrode 52 of the diode transistor M2 may be formed incommon with the gate electrode 51 of the switching transistor M1. Thesource region of the diode transistor M2 is coupled to the data line 110through the contact hole h1, the drain region of the diode transistor M2is coupled to the drain region of the driving transistor M3 through thesemiconductor layer 30.

The driving transistor M3 may be formed at an intersection between thescan line 120 and the power line 130. The gate electrode 54 of thedriving transistor M3 is coupled to the drain region of the switchingtransistor M1 through the contact holes h3 and h2, the source region ofthe driving transistor M3 is coupled to the power line 130 through acontact hole h4, and the drain region of the driving transistor M3 iscoupled to a source region of the emission transistor M4 through thesemiconductor layer 30.

The emission transistor M4 may be formed across the emission controlline 140. The gate electrode 53 of the emission transistor M4 may beformed by a portion of the emission control line 140 and the drainregion of the emission transistor M4 may be coupled to the organic lightemitting element OLED.

The capacitors C1 and C2 may overlap with the power line 130 in thevicinity of a long side 201 of the organic light emitting element OLED.A first electrode 36 of the first capacitor C1 is coupled to the boostcontrol line 150 and a first electrode 37 of the second capacitor C2 iscoupled to the power line 130. Second electrodes 56 and 57 of thecapacitors C1 and C2 may be formed as a common layer. And, the secondelectrodes 56 and 57 of the capacitors C1 and C2 may be coupled to thegate electrode 54 of the driving transistor M3 through a contact holeh7. At this time, the second electrodes 56 and 57 of the capacitors C1and C2 may be coupled to the drain region of the switching transistor M1and the gate electrode 54 of the driving transistor M3 through theconductive layer 135 contacted on the gate electrode 54 of the drivingtransistor M3.

Now, an interlayer structure of the display panel having the above pixelcircuit and a method for fabricating the organic EL display panel willbe described.

The organic EL display panel according to an exemplary embodiment of thepresent invention may have a blocking layer 20, which may be made fromsilicon oxide, silicon nitride, or other similar substances, that may beformed on an insulation substrate 10.

Thereafter, the semiconductor layer 30 for the switching transistor M1,the driving transistor M3, the diode transistor M2 and the emissiontransistor M4 formed as a common layer on the blocking layer 20. Thesemiconductor layer 30 is formed with polycrystalline silicon. Thesemiconductor layer 30 may be formed by a laser crystalline method on asemiconductor film having an amorphous structure. Also, the firstelectrodes 36 and 37 of the first and second capacitors C1 and C2 may beformed on the blocking layer 20.

Thereafter, a gate insulation film 40, which may be made from siliconoxide, silicon nitride, or other like materials, may be formed on thesemiconductor layer 30. A hole h9 exposing the first electrode 36 of thefirst capacitor C1.

Thereafter, the scan line 120, which may include a conductive film madefrom conductive material with low resistance such as aluminum oraluminum alloy, and the gate electrodes 51, 52, 53 and 54 of thetransistors M1, M2, M4 and M3, may be formed on the gate insulation film40. The emission control line 140 and the boost control line 150 may beformed on the gate insulation film 40 using the same material as thescan line 120 and the gate electrodes 51, 52, 53 and 54. Also, thesecond electrodes 56 and 57 of the capacitors C1 and C2 may be on thegate insulation film 40.

In more detail, the gate electrode 51 of the switching transistor M1 andthe gate electrode 52 of the diode transistor M2, which may be formed ina branch shape, may be coupled to the scan line 120 and overlap thechannel region 31 a of the switching transistor M1 and the channelregion 32 a of the diode transistor M2, respectively. Additionally, thegate electrode 53 of the emission transistor M4 may be separated fromthe scan line 120 and overlap with the channel region 33 a of thesemiconductor layer 30. At this time, the gate electrode 53 of theemission transistor M4 may be formed by a portion of the emissioncontrol line 140 on the same layer. The emission control line 140 mayform the gate electrode 53 of the emission transistor M4 while extendingin a row direction and overlapping the channel region 33 a of theemission transistor M4. Also, the gate electrode 54 of the drivingtransistor M3 may be separated from the scan line 120 and overlap withthe channel region 34 a of the driving transistor M3.

Thereafter, the source regions 31 b, 32 b, 33 b and 34 b, drain regions31 c, 32 c, 33 c and 34 c for the transistors M1, M2, M4 and M3 arerespectively formed. The source regions 31 b, 32 b, 33 b and 34 b andthe drain regions 31 c, 32 c, 33 c and 34 c may be doped with p-type orn-type impurities according to driving conditions.

Thereafter, a first interlayer insulation film 60 may be formed.

Contact holes h3 and h7 may be formed in the first interlayer insulationfilm 60 to couple the second electrodes of the capacitors C1 and C2 tothe gate electrode 54 of the driving transistor M3, And, contact holeh1, h2, h4, h5, h8 may be formed in the gate insulation 40 and the firstinterlayer film 60.

Thereafter, the conductive layer 135, the data line 110, the power line130, a connection electrode 71 of the emission transistor M4 are formedon the first interlayer insulation film 60.

The conductive layer 135 may have a “L”-like shape connected to thecapacitors C1 and C2 through the contact hole h7, starting from thedrain region 31 c of the switching transistor M1, passing over the gateelectrode 54 of the driving transistor M3, and traversing the emissioncontrol line 140. Accordingly, the capacitors and the driving transistorM3 form a node at the gate electrode 54 of the driving transistor M3through the conductive layer 135 formed on the first interlayerinsulation film 60.

At this time, the upper side of the gate electrode 54 of the drivingtransistor M3 is empty since the source and drain regions 34 c of thedriving transistor M3 are coupled to the power line 130 and the diodetransistor M2, respectively, via the semiconductor layer 30, rather thanvia the conductive layer 135. Namely, the driving transistor M3 has thedrain region 34 c of the semiconductor layer 30 that may be coupled tothe drain region 32 c of the diode transistor M2, which may be formed ofthe same material and in the same layer as the drain region 34 c of thedriving transistor M3. Further, the source region 34 b of the drivingtransistor M3 may be coupled to the power line 130 through the contacthole h4 in the gate insulation film 40 and the first interlayerinsulation film 60.

Accordingly, the conductive layer 135 may be contacted on the gateelectrode 54 of the driving transistor M3, and it couples the drainregion 31 c of the switching transistor M1 and the gate electrode 54 ofthe driving transistor M3 to each other through the contact holes h2 andh3. Additionally, since the conductive layer 135 couples the gateelectrode 54 of the driving transistor M3 to the second electrodes 56and 57 of the capacitors C1 and C2 through the contact hole h7, a nodehaving the shortest distance where the switching transistor M1, thedriving transistor M3 and the capacitors intersect at the gate electrode54 of the driving transistor M3 may be formed. Accordingly, a space inwhich the pixel circuit is disposed may be reduced, and relatively, aspace in which the organic light emitting element OLED is disposed maybe widened, thereby increasing the panel's aperture ratio. (See FIG. 9).

The data line 110 may be formed on the first interlayer insulation film60, and it may extend in a column direction. Further, it may be coupledto the source region 31 b of the switching transistor M1 and the sourceregion 32 b of the diode transistor M2 through the contact hole h1passing through the first interlayer insulation film 60 and the gateinsulation film 40.

Similarly, the power line 130 may be formed on the first interlayerinsulation film 60, and it may extend in the column direction. Further,it may be coupled to the source region 34 b of the driving transistor M3through the contact hole h4 passing through the first interlayerinsulation film 60 and the gate insulation film 40.

Additionally, the connection electrode 71 of the emission transistor M4may be formed with the same material and in the same layer as the dataline 110 and the power line 130. Namely, the connection electrode 71 maybe coupled to the drain region 33 c of the emission transistor M4through the contact hole h5 passing through the first interlayerinsulation film 60 and the gate insulation film 40.

Thereafter, a second interlayer insulation film 80, which may be madefrom silicon nitride, silicon oxide, organic insulation material orother like substances, may be formed on the data line 110, the powerline 130, and the connection electrode 71 of the emission transistor M4.The second interlayer insulation film 80 may have the contact hole h6for coupling the organic light emitting element OLED to the connectionelectrode 71 of the emission transistor M4.

Thereafter, a pixel electrode 81 of the organic light emitting elementOLED, which may be formed on the second interlayer insulation film 80,may be coupled to the connection electrode 71 of the emission transistorM4 through the contact hole h6. The pixel electrode 81 may be formedwith reflective material, such as aluminum or silver alloy.Alternatively, the pixel electrode 81 may be formed with transparentmaterial such as indium tin oxide (ITO) or indium zinc oxide (IZO). Thepixel electrode 81 made from transparent conductive material may beapplied to an organic EL display utilizing the bottom emission method todisplay an image at the rear of the display panel. The pixel electrode81 made from non-transparent conductive material may be applied to anorganic EL display utilizing the top emission method to display an imageat the front of the display panel.

Thereafter, a partition wall 83, which may be made from organicinsulation material to seperate organic emitting cells from each other,may be formed on the second interlayer insulation film 80. The partitionwall 83 may surround the pixel electrode 81 to define a region of theorganic light emitting element OLED. The partition wall 83 serves as alight-blocking film by exposing and developing a photosensitive agentincluding a black paint. Also, the partition wall 83 may simplify a filmforming process. An organic emission layer 85 may be formed on the pixelelectrode 81. The organic emitting element OLED comprises the organicemission layer 85, which may emit red, green or blue light.

Thereafter, a buffer layer 90 may be formed on the organic emissionlayer 85 and the partition wall 83. The buffer layer 90 may be omitted.

Thereafter, a common electrode 95 may be formed on the buffer layer 90.The common electrode 95 may be made from transparent conductive materialsuch as ITO or IZO. The common electrode 95 may also be made fromreflective metal, such as aluminum.

Additionally, an auxiliary electrode (not shown) may be formed with alow-resistance metal to enhance the conductivity of the common electrode95. The auxiliary electrode may be formed between the common electrode95 and the buffer layer 90, or it may be formed on the common electrode95. Also, the auxiliary electrode is preferably formed in the form of amatrix along the partition wall 83 so that it does not overlap with theorganic emission layer 85.

As is apparent from the above description, with the organic EL displaypanel according to exemplary embodiments of the present invention, sincethe current flowing through the organic light emitting element OLED maybe controlled with a large value of current, driving the display panelby precise current programming is possible. Also, a deviation inbrightness between pixels may be alleviated by compensating for adeviation in threshold voltage or mobility between pixels, which mayoccur in processes for fabricating transistors.

Particularly, compactly arranging transistors within a small spacewidens a space in which the organic light emitting element is disposed,thereby increasing the display panel's aperture ratio.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An organic electro luminescent (EL) display panel comprising at leastone pixel circuit for driving each pixel, wherein the pixel circuitcomprises: at least one organic light emitting element per pixel; atleast two thin film transistors per pixel; and at least one capacitorper pixel, wherein the thin film transistors respectively comprise agate electrode and a semiconductor layer in which a channel region, asource region and a drain region are formed; and wherein a conductivelayer contacted on the gate electrode of one of thin film transistors iscoupled to another thin film transistor and at least one capacitor. 2.The organic EL display panel of claim 1, wherein the semiconductor layerof the thin film transistors is formed as a common layer.
 3. The organicEL display panel of claim 1, further comprising: a data line and a powerline in parallel to each other; and a scan line, an emission controlline, and a boost control line in parallel to each other andintersecting the data line and the power line, wherein a first region isdefined by the data line, the power line, the scan line, and theemission control line; wherein a second region is defined by the boostcontrol line, the emission control line, the data line, and the powerline, and wherein the pixel circuit is formed in the first region andthe second region.
 4. The organic EL display panel of claim 3, whereinthe at least two thin film transistors are formed in the first region;and wherein the at least one capacitor and the organic light emittingelement are formed in the second region.
 5. The organic EL display panelof claim 3, wherein the pixel circuit includes: a first thin filmtransistor and a second thin film transistor for transporting a datacurrent from the data line in response to a selection signal from thescan line; a third thin film transistor for supplying a driving currentto the organic light emitting element; a fourth thin film transistor fortransporting the driving current from the third thin film transistor tothe organic light emitting element; a first capacitor to be charged to afirst voltage corresponding to the data current from the first thin filmtransistor; and a second capacitor coupled between the first capacitorand the boost control line for changing the first voltage of the firstcapacitor to a second voltage, wherein the third thin film transistor isdiode-coupled while the data current is transported from the first thinfilm transistor and the second thin film transistor.
 6. The organic ELdisplay panel of claim 5, wherein the first thin film transistor isformed in a region where the data line and the scan line intersect,wherein the second thin film transistor is formed in a region where thedata line and the emission control line intersect, wherein the thirdthin film transistor is formed in a region where the scan line and thepower line intersect, and wherein the fourth thin film transistor isformed across the emission control line.
 7. The organic EL display panelof claim 6, wherein a drain region of the first thin film transistor iscoupled to the third thin film transistor and the capacitors by theconductive layer.
 8. The organic EL display panel of claim 7, whereinthe conductive layer has a “L” like shape starting from the drain regionof the first thin film transistor, passing over a gate electrode of thethird thin film transistor, and traversing the emission control line. 9.The organic EL display panel of claim 8, wherein the gate electrode ofthe third thin film transistor is coupled to the drain region of thefirst thin film transistor and the capacitors via the conductive layerthrough a contact hole exposing the gate electrode of the third thinfilm transistor.
 10. The organic EL display panel of claim 9, whereinthe third thin film transistor is coupled to the second thin filmtransistor through the semiconductor layer, and wherein thesemiconductor layer extends across the emission control line to thesecond region to form a drain region of the fourth thin film transistor.11. The organic EL display panel of claim 6, wherein the conductivelayer is formed on a same insulation film and is formed of a samematerial as the data line and the power line.